Depth profiling of degradation of multilayer photovoltaic backsheets after accelerated laboratory weathering: Cross-sectional Raman imaging

Abstract

Understanding of thedurability of each individual layer and their interfaces in a multilayered photovoltaic (PV)backsheetis critical to the design and selection of materials for making reliable andhigh performance PV modules. In this study, Raman imaging was used to depth profile the chemical degradation of a multilayer commercial backsheetfilm exposed to ultraviolet (UV) radiationat 85°C, 5% relative humidity (RH, dry) and 85°C,60% RH (humid)on the NIST (National Institute of Standards and Technology) SPHERE(Simulated Photodegradation via High Energy Radiant Exposure).The backsheetfilm wasa multipartlaminate comprising of a pigmented polyethylene terephthalate (PET)-based outer layer, PET core layer and three ethylene vinyl acetate (EVA) layers having different vinyl acetate(VA) contents, along with two inner adhesive layers between PET outer and PET core layers, and PET core and EVA layers. Cross-sectional samples were prepared by cryo-microtomyfor various characterizations.The multilayer structureswere examined by laser scanning confocal andatomic force microscopies, while their chemical degradation profiles were obtained by Raman spectroscopic imaging. Non-uniform degradation was observed in the agedbacksheet film, and both UV and moisture appeared tosignificantly affect the degradation profiles of the multilayers.Severe degradation, indicated by high fluorescence,occurredin the outermost region of the pigmented PET outer layer, and the degradation gradient extended to approximately 20 µm to the bulk. It was also found thattheinner adhesive layerswere severelydeterioratedunder moist condition, indicating thatthe long-termadhesion between the layers could be a major area of concern for multilayer backsheets used in a humid environment. The relationship between the sharp 3 (non-uniform) degradation profile, resultant internal stress, and ultimate failures (cracking and delamination)was discussed as well.

A novel test method for quantifying cracking propensity of photovoltaic backsheets after ultraviolet exposure

“…Despite the retention of bulk mechanical properties after aging, WPET surface cracking was observed solely in samples aged with UV exposure, but not unaged samples or samples aged in the dark conditions. Surface embrittlement as a result of UV photodegradation is believed to be the main cause of the cracking . Figure displays the progressive development of surface channel cracks (fragmentation) under uniaxial stretching for a sample aged in UV humid conditions for 255 hours.…”

“…Commercially available freestanding PPE backsheets were characterized in the present work. The PPE backsheet comprises 7 laminated layers, including a 55‐μm pigmented white PET (WPET) outer layer, a 126‐μm PET core layer, a 25‐μm EVA inner layer, a 50‐μm pigmented EVA layer, a 25‐μm EVA outer layer, and two 6.5 and 8‐μm adhesive layers; a detailed material composition of each layer was reported in our previous work . The WPET layer is designed to be weathering resistant and is equibiaxially stretched in the manufacturing process, while the EVA layers are engineered to adhere to module encapsulant and are essentially oriented in the machine direction.…”

“…Rectangular PPE sheets (70 mm x 104 mm) were exposed to highly uniform UV radiation at precisely controlled temperature and relative humidity (RH) on the National Institute of Standards and Technology (NIST) Simulated Photodegradation via High Energy Radiant Exposure (SPHERE) with the WPET side facing the radiation source . Accelerated aging was introduced with UV irradiance intensity of approximately 140 W/m 2 (300 to 400 nm).…”

“…Surface cracking behavior of backsheet relies crucially on the integrity of the outermost weathering layer. As the surface of the weathering layer gradually degrades because of adverse environmental factors, an embrittled surface layer is formed . To quantify the propensity of backsheet cracking, external uniaxial load is applied to provide applied strain ( ε a ) on the backsheet, and simultaneous surface monitoring was performed under microscope, as shown in Figure .…”

“…As the backsheets gradually degrade and lose their mechanical integrity over time, surface cracks occur and propagate, leading to concerns of safety because of the increased moisture and oxygen ingress . The cracking has been postulated to be the result of embrittlement of the surface layer caused by prolonged ultraviolet (UV) exposure even though these polymeric materials are UV‐stabilized and/or pigmented …”